Internal combustion engines employing advanced combustion processes are being developed in order to comply with evermore stringent fuel economy requirements and restrictions on emissions such as CO2, NOx, CO, hydrocarbons, and particulate matter. One such internal combustion engine employing an advanced combustion process is known as a gasoline direct injection compression ignition (GDCI) internal combustion engine which provides the high efficiency of a diesel internal combustion engine while using regular unleaded gasoline as the fuel. The GDCI process relies on controlled autoignition of gasoline fuel in a compression ignition engine. In the GDCI process, no gasoline is injected into the combustion chamber during the intake stroke. Rather, gasoline is injected into the combustion chamber late in the compression stroke. The gasoline and air rapidly mix and compression ignites the mixture in a controlled heat release process.
In order to achieve maximum fuel efficiency in a GDCI internal combustion engine, autoignition must occur over a wide range of operating loads, speeds, and temperatures. This includes using GDCI during cold starts, warm-up periods, and at light loads when autoignition using gasoline is very difficult to sustain. In order for autoignition to occur under these conditions in a GDCI internal combustion engine, special engine subsystems are needed to control conditions within the combustion chamber including pressure, temperature, air-fuel ratio, burned gas dilution, and charge motion. Additional heat may need to be added to the combustion chamber in order for the temperature therein to be sufficient to achieve autoignition. One method for introducing heat into the combustion chamber is to use negative valve overlap. When using negative valve overlap, camshaft phasers with large angular displacement are used to adjust the timing of the intake and exhaust valves to trap exhaust constituents in the combustion chamber by closing the exhaust valves prior to the end of the exhaust stroke. Conventional two-step actuation of the intake valves and exhaust valves may also be simultaneously employed to implement valve lift profiles that have been optimized for operation during negative valve overlap. However, such camshaft phasers and two-step actuation of the intake valves and exhaust valves can add significant cost and complexity to the valve train system. Additionally, using negative valve overlap negatively affects pumping work of the internal combustion engine and heat of the trapped exhaust constituents can be lost to the walls of the combustion chamber.
Another method for introducing heat into the combustion chamber is to open the intake valve during the exhaust stroke. This allows hot exhaust constituents into the intake system of the internal combustion engine which are then reintroduced into the combustion chamber during the subsequent intake stroke. This method can produce large amounts of hot residuals for mixture heating, but has the disadvantage of heating the walls of the intake port and runner.
Yet another method for introducing heat into the combustion chamber which may be more advantageous than using negative valve overlap or opening the intake valve during the exhaust stroke is to use exhaust rebreath. When using exhaust rebreath, hot exhaust constituents are introduced into the combustion chamber through the exhaust valve during the intake stroke. Exhaust rebreath does not compromise engine efficiency because exhaust constituents entering the cylinder during the intake stroke increases the pressure within the combustion chamber, thereby reducing the pumping loop. Using the exhaust rebreath method during cold starts may also reduce the time required to elevate the temperature of a catalyst in an exhaust treatment device sufficient to allow the catalyst to convert the exhaust species to less harmful constituents. This is because the exhaust temperature will be higher during exhaust rebreathing due to the decrease in intake air flow. The exhaust rebreath method is also helpful in maintaining temperature of the catalyst during deceleration conditions when fuel is shut off and also during low load conditions. In each of these conditions, the temperature of the catalyst may fall below the threshold required for the catalyst to convert the exhaust species to less harmful constituents.
U.S. Pat. No. 7,308,872 which is assigned to Applicant and incorporated herein by reference in its entirety teaches a valve train system which is useful in homogeneous charge compression ignition (HCCI) internal combustion engines. HCCI internal combustion engines mix air and fuel together in the intake stroke and compression of the mixture during the compression stroke will cause autoignition. U.S. Pat. No. 7,308,872 teaches an exhaust camshaft lobe for opening and closing an exhaust lobe of the valve train system to expel exhaust constituents from the combustion chamber. A portion of the exhaust lobe profile allows the exhaust lobe to be held open for a brief time period of the intake stroke to allow a small amount of exhaust rebreath. In this way, exhaust constituents are allowed to enter the combustion chamber through the exhaust valve. When used with a camshaft phaser, the duration of time the exhaust valve is open during the intake stroke can be varied. However, since the portion of the exhaust lobe profile that causes the exhaust rebreath is part of the main exhaust lobe that allows exhaust constituents to exit the combustion chamber, there is always some amount of rebreath of exhaust constituents. Additionally, the rebreath event must occur at the beginning of the intake stroke while some internal combustion engines may benefit from the rebreath event occurring near the end of the intake stroke.
What is needed is a valve train system that allows for varying amounts of rebreath of exhaust constituents. What is also needed is a valve train system that allows the rebreath of exhaust constituents to be discontinued when desired. What is also needed is a valve train system that allows the rebreath of exhaust constituents to occur near the end of the intake stroke.